The present invention relates generally to data processing systems and, more particularly, to the use of storage devices, such as nonvolatile memory systems in data processing systems.
Recently, the number of devices using nonvolatile memories has increased. For example, an MP3 player, a digital camera, a mobile phone, a camcorder, a flash card, and a solid state disk (SSD) are examples of devices that use nonvolatile memories as storage devices.
As more devices use nonvolatile memories as storage devices, the capacity of the nonvolatile memory is generally increasing. One method for increasing memory capacity is use of a so-called multi level cell (MLC) method in which a plurality of bits is stored in one memory cell.
A two-bit MLC may be programmed to have any one of four states 11, 01, 10 and 00 according to distribution of a threshold voltage. Over time, however, cells may be stressed through frequent programming operations and erase operations along with temperature variation. The threshold voltage levels for cells that are programmed and erased more frequently may vary more than the threshold voltage levels for cells that are programmed and erased less frequently. This may result in the threshold voltage levels shifting, which can cause errors in reading the data contained therein.